Using the polymerase chain reaction (PCR) for hepatitis B virus (HBV) DNA and an enzyme linked immunosorbant assay for the antibody to the hepatitis C virus (anti-HCV), we studied the serum samples of 18 patients with posttransfusion non-A, non-B (NANB) hepatitis in a prospective study. They were all negative for both the hepatitis B surface antigen (HBsAg) and the IgM antibody to the hepatitis B core antigen (IgM anti-HBc) by radioimmunoassay and were all negative for hepatitis B virus (HBV) DNA by dot blot hybridization. Eleven patients were positive for anti-HCV in the posttransfusion sera and 2 were already positive in the pretransfusion sera. Therefore, seroconversion of anti-HCV was noted in 9 patients. Two of these 18 patients were positive for HBV DNA by PCR in the posttransfusion sera and one of the 2 patients was also positive in the pretransfusion serum sample. The patient positive for HBV DNA in both pre and posttransfusion sera seroconverted to anti-HCV, while the patient positive for HBV DNA only in the posttransfusion sera was not seroconverted. We conclude that HBV plays minimal role in patients with posttransfusion NANB hepatitis in Taiwan even studied by PCR.